Paleointensity during the Laschamp and Blake geomagnetic excursions

Abstract

Full vector information of the Earth’s magnetic field on a geological time scale is essential to put constraints on geodynamo models. Excursions of the geomagnetic field show deviating directions and a strong decrease in intensity and they are thought to be caused by the same geodynamo processes as reversals. In this study we attempt to assess the properties of the paleofield during the Laschamp and Blake excursions by investigating 15 lava flows (28.6 - 154.9 ka) from Mount Etna on Sicily. Directions of the magnetization are obtained and the paleointensity is acquired by three different absolute methods (Thellier-Thellier, the original multispecimen protocol, and the domain state corrected multispecimen protocol) and one relative method (pseudo-Thellier).

Five sites were selected based on the preliminary measurements and yielded technically good results with all paleointensity methods. The Thellier-Thellier method yielded the highest paleointensity for these sites, which are taken as an upper bound. The MSP-DB method produced lower values that are taken as a lower bound for the paleointensity. Results of the MSP-DSC method were not reliable as they were clearly affected by alteration during subsequent heating steps. The calibration formula of the pseudo-Thellier method seemed not applicable to these specimens. A new calibration based on one of the absolute methods, however, could not be made. Resulting paleointensities range from 30.9 μT to 122.3 μT and could not be correlated to either the Laschamp or the Blake excursion. Two other sites might be related to the Laschamp excursion based on the age and direction, but a paleointensity could not be obtained.